Since the compound in the present invention is synthesized by reacting .alpha.-L-aspartyl-L-phenylalanine methyl ester with 3,3-dimethylbutyl aldehyde as described in the publication, for example, Tokuhyouhei-JP-A-8-503206, EP Patent Publication WO95/30689, or etc., it is difficult to avoid the mixing of unreacted .alpha.-L-aspartyl-L-phenylalanine methyl ester into N-(3,3-dimethylbutyl)-.alpha.-L-aspartyl-L-phenylalanine methyl ester. Accordingly, it is most important to remove .alpha.-L-aspartyl-L-phenylalanine methyl ester for the purification of N-(3,3-dimethylbutyl)-.alpha.-L-aspartyl-L-phenylalanine methyl ester. It is described in the publication of Tokuhyouhei-JP-A-503206 that the compound in the present invention can be purified by a standard technique such as recrystallization and chromatography. The chromatography is not an appropriate technique for an industrial purification method. And, the recrystallization is in most common use for an industrial purification method, and however since the desired compound is dissolved in the mother liquor, and the dissolved compound is lost away, it is difficult to say that it is an efficient technique in view of the yield. Accordingly, a recovering method from the mother liquor is employed industrially by necessity to improve the yield. In the meantime, there is little difference between the solubility of .alpha.-L-aspartyl-L-phenylalanine methyl ester and the solubility of N-(3,3-dimethylbutyl)-.alpha.-L-aspartyl-L-phenylalanine methyl ester at a temperature of from 5 to 25.degree. C. in the water which is usually employed for the solvent in the recrystallization of peptides and thus an amount of N-(3,3-dimethylbutyl)-.alpha.-L-aspartyl-L-phenylalanine methyl ester nearly equal to that of .alpha.-L-aspartyl-L-phenylalanine methyl ester is lost in the mother liquor through the recrystallization step. In addition, a recovering method from the mother liquor is not described in any specifications of the above-mentioned 2 prior patent publications. Typically, the object (desired) compound may be recovered by concentrating and cooling the mother liquor to crystallization, and however this recovering method requests burdensome steps or operations on the concentration, crystallization and separation, and further is not efficient also from the recovery rate (yield) thereof, because the object compound is lost in the mother liquor.